The first, is that the time and date set on your computer is incorrect. Check to make sure that the time, date (especially year) are correct.
The second is that the libraries which allow encryption have been damaged and need to be re-registered. To re-register them, click Start, then Run, and enter the following: regsvr32 softpub.dll then click OK.
and do the same for the following: regsvr32 wintrust.dll regsvr32 initpki.dll regsvr32 MSXML3.dll
The third is that you might be using a third-party software firewall which is blocking Messenger. In this case, simply set Messenger to access to the Internet once again. Failing that, completely uninstall the firewall entirely to see if that fixes the problem. Then reinstall it after you've confirmed that was the problem as more than likely it will re-configure itself properly.